Mastodon has microblogging features similar to Twitter. Each user is a member of a specific Mastodon server, known as an "instance" of the software, but can connect and communicate with users on other instances as well. Users post short messages called "toots" for others to see, subject to the adjustable privacy settings of the user and their particular instance. The Mastodon mascot is a brown or grey Proboscidean sometimes depicted using a tablet or smartphone.

The software seeks to distinguish itself from Twitter through its orientation towards independently operated small communities. Thus it has a community-based, rather than top-down, moderation and service operation, based on cooperation without direct dependence.[improper synthesis?] Like Twitter, Mastodon supports direct, private messages between users, but unlike "tweets" posted on Twitter, Mastodon’s "toots" can be either private to the user, private to the user's followers, public on a specific instance, or public across a network of instances.

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Mastodon servers run social networking software that is capable of communicating either via the OStatus protocol, or also the newer ActivityPub standard. A Mastodon user can therefore interact with users on any other server in the Fediverse that supports these.

Mastodon approximates the microblogging user experience of Twitter, where users post short-form status messages for others to see. On Mastodon, these messages can include up to 500 text-based characters, an extension of Twitter's 280-character limit (originally 140),[3] and posts are called "toots" instead of "tweets", as is the case on Twitter.[4]

Users join a specific Mastodon server, known as an "instance", rather than a single website or application. The instances are connected as nodes in a network, and each server can administrate its own rules, account privileges, and whether to share messages to and from other instances. The flagship instance, Mastodon.social, had about 42,000 users as of early April 2017. Other instances are based on communal interests, such as Internet memes, Minecraft, or technology.[4] The global use has risen from 766,500 users as of August 1, 2017,[5] to 1 million users on December 1, 2017.[6]

The service includes a number of privacy features. Each message has a variety of privacy options available, and users can choose whether the post is public or private. Public messages display on a global feed, known as a timeline, and private messages are only shared on the timelines of the user's followers. Messages can also be marked as unlisted from timelines or direct between users. Users can also mark their accounts as completely private. In the timeline, messages can display with an optional "content warning" feature, which requires readers to click on the content to reveal the rest of the message. Mastodon instances have used this feature to hide spoilers, trigger warnings, and not safe for work (NSFW) content, though some accounts use the feature to hide links and thoughts others might not want to read.[4]

Mastodon aggregates messages in local and federated timelines. The local timeline shows messages from users on a singular instance, while the federated timeline shows messages across all participating Mastodon instances. Users can communicate across connected Mastodon instances with usernames similar in format to full email addresses.[4]

In early 2017, journalists[Like whom?] distinguished Mastodon from Twitter for its approach to combating harassment, one of Twitter's largest issues.[dubious – discuss][4] Mastodon uses community-based moderation, in which each instance can limit, or filter out undesirable types of content. For example, the flagship instance, Mastodon.social, bans content that is illegal in Germany or France, including Nazi symbolism, Holocaust denial and discrimination. Several other instances do this too. Instances can also choose to limit, or filter out messages with disparaging content. Mastodon's founder Eugen Rochko believes that small, close communities would police toxic behavior more effectively than a large company's small safety team.[citation needed] Users can also block and report others to administrators, much like on Twitter.[7][4]
In September 2018, with the release of version 2.5, that features redesigned public profile pages, Mastodon marked its 100th release.[8] Then, at the end of October, Mastodon 2.6 came out, introducing the possibilities of verified profiles and live, in-stream link previews for images and videos.[9]

While Mastodon was first released in October 2016, the service began to expand in late March and early April 2017.[14]The Verge wrote that the community at this time was small and that it had yet to attract the personalities that keep users at Twitter.[4] In November 2017 artists, writers and entrepreneurs such as Chuck Wendig, John Scalzi, Melanie Gillman and later John O'Nolan joined in, while Mastodon had reached 1 million accounts on December 1, 2017.[6][15][16][17][18]
Another spike in popularity came in March, through April of 2018, due to the concerns about user privacy raised by the #deletefacebook effort.[19]

Mastodon, along with a number of other alternative social media sites, saw a large uptick in membership, gaining thousands of new members in the period of a few hours compared to dozens in days prior,[20] following Tumblr's announcement of intent in early December 2018 to ban all sensitive content from their site.[21]